This invention relates to acoustic ink printing and, more particularly, to acoustic ink printing with hot melt inks.
Acoustic ink printing is a promising direct marking technology because it does not require the nozzles of the small ejection orifices which have been a major cause of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have experienced.
It has been shown that acoustic ink printers that have print heads comprising acoustically illuminated spherical or Fresnel focusing lenses can print precisely positioned picture elements (pixels) at resolutions which are sufficient for high quality printing of complex images. See, for example, the co-pending and commonly assigned U.S. Pat. No. 4,751,529 on xe2x80x9cMicrolenses for Acoustic Printingxe2x80x9d, and U.S. Pat. No. 4,751,530 on xe2x80x9cAcoustic Lens Arrays for Ink Printingxe2x80x9d to Elrod et al., which are both hereby incorporated by reference. It also has been found that the size of the individual pixels printed by such a printer can be varied over a significant range during operation.
Although acoustic lens-type droplet emitters currently are favored, there are other types of droplet emitters which may be utilized for acoustic ink printing, including (1) piezoelectric shell transducers or an acoustic lens-type drop emitter, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a xe2x80x9cLiquid Drop Emitter,xe2x80x9d and (2) interdigitated transducer (IDT""s), such as described in commonly assigned U.S. Pat. No. 4,697,195 on xe2x80x9cNozzleless Liquid Droplet Ejectorsxe2x80x9d, to Quate et al. Furthermore, acoustic ink printing technology is compatible with various print head configurations; including (1) single emitter embodiments for raster scan printing, (2) matrix configured arrays for matrix printing, and (3) several different types of page width arrays, ranging from (I) single row, sparse arrays for hybrid forms of parallel/serial printing, to (ii) multiple row staggered arrays with individual emitters for each of the pixel positions or addresses within a page width address field (i.e., single emitter/pixel/line) for ordinary line printing.
For performing acoustic ink printing with any of the aforementioned droplet emitters, each of the emitters launches a converging acoustic beam into a pool of ink, with the angular convergence of the beam being selected so that it comes to focus at or near the free surface (i.e., the liquid/air interface) of the pool. Moreover, controls are provided for modulating the radiation pressure which each beam exerts against the free surface of the ink. That permits the radiation pressure of each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension, whereby individual droplets of ink are emitted from the free surface of the ink on command, with sufficient velocity to deposit them on a nearby recording medium.
Hot melt inks have the known advantages of being relatively clean and economical to handle while they are in a solid state and of being easy to liquify in situ for the printing of high quality images. Another advantage lies in that there is no need to dry paper (as in water-based inks) and no bleeding of different colors. These advantages are of substantial value for acoustic ink printing, especially if provision is made for realizing them without significantly complicating the acoustic ink printing process or materially degrading the quality of the images that are printed.
A drawback of using hot melt inks in acoustic ink printing is that such inks have a relatively high viscosity. Particularly, the inks can be in the form of, but are not limited to, a solid material at room temperature and are liquefied at elevated temperatures to achieve a viscosity of approximately 5-10 centipoise (cp). When hot melt inks are used to fill in the complete focal zone of an acoustic lens, as is the case with a standard acoustic ink printer, significant acoustic attenuation occurs in the focal path. This will, therefore, require that the input power to a printer be raised to a much higher level to overcome the attenuation, which in turn results in increased power consumption and stress on the system. When too much of an acoustic wave is attenuated, it is not possible to emit ink drops, or undesirable undeformed, or misdirected ink drops with very low velocity are generated.
FIG. 1 provides a view of an exemplary acoustic ink printing element 10 to which the present invention may be applied. Of course, other configurations may also have the present invention applied thereto.
As shown, the element 10 includes a glass layer 12 having an electrode layer 14 disposed thereon. A piezoelectric layer 16, preferably formed of zinc oxide, is positioned on the electrode layer 14 and an electrode 18 is disposed on the piezoelectric layer 16. Electrode layer 14 and electrode 18 are connected through a surface wiring pattern representatively shown at 20 and cables 22 to a radio frequency (RF) power source 24 which generates power that is transferred to the electrodes 14 and 18. On a side opposite the electrode layer 14, a lens 26, preferably a concentric Fresnel lens, is formed. Spaced from the lens 26 is a liquid level control plate 28, having an aperture 30 formed therein. Ink 32 is retained between the liquid level control plate 28, having an aperture 30 formed therein. Ink 32 is retained between the liquid level control plate 28 and the glass layer 12, and the aperture 30 is aligned with the lens 26 to facilitate emission of a droplet 34 from ink surface 36. Ink surface 36 is, of course, exposed by the aperture 30.
The lens 26, the electrode layer 14, the piezoelectric layer 16, and the electrode 18 are formed on the glass layer 12 through known photolithographic techniques. The liquid level control plate 28 is subsequently positioned to be spaced from the glass layer 12. The ink 32 is fed into the space between the plate 28 and the glass layer 12 from an ink supply (not shown).
A droplet emitter is disclosed in commonly assigned U.S. Patent to Hadimioglu et al. U.S. Pat. No. 5,565,113, entitled xe2x80x9cLithographically Defined Ejection Unitsxe2x80x9d and in commonly assigned U.S. Pat. No. 5,591,490 to Quate entitled xe2x80x9cAcoustic Deposition of Material Layersxe2x80x9d, both hereby incorporated by reference.
While the above concepts provide advantages, drawbacks exist. Particularly, an ink print head in which the above device is implemented is required to perform repetitive tasks at a high level of frequency. Further, such a device is implemented in a hostile environment with large fluctuations in heat and operating parameters. Therefore, there is a concern as to the robustness of the liquid cell when used in a print head. Particularly, there are concerns that use of the capping structure may be insufficient to maintain the integrity of the liquid cell. Another drawback is the difficulty of filling the liquid cell with a layer of liquid so as to maintain the liquid cell free from air pockets or bubbles which would disrupt the acoustic waves traveling through the liquid cell.
In view of the above, it is considered desirable to develop an emitter in an acoustic ink print head which can emit hot melt ink. The print head should be robust and able to operate with a high degree of reliability, is economical to make, and is manufactured consistent with fabrication techniques of existing acoustic ink print heads.
The present invention describes bi-layer structures integrated into individual emitters of an acoustic ink print head which enables the print head to emit droplets of high viscosity fluid such as hot melt inks. The bi-layer structure is provided above the glass substrate but below the ink surface of the acoustic ink emitter and is used to avoid attenuation of acoustic waves which would occur in a reservoir full of high-viscosity fluids. Also disclosed is a method of fabricating the bi-layer structures.
A benefit of the present invention is an improvement in the accuracy and functionality of an acoustic ink print head which is intended to emit droplets of a high-viscosity fluid such as hot melt inks.
Another benefit of the present invention is that such a structure is compatible with present fabrication techniques for acoustic ink print heads wherein emitters are beneficially lithographically defined and formed using conventional thin-film processing (such as vacuum deposition, epitaxial growth, wet etching, dry etching, and plating).
These together with other objects of the invention, along with the various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects obtained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention.